The invention relates to a galvanic solid electrolyte sensor for measuring gaseous anhydrides, such as CO.sub.2, SOx, and NOx and is based on a sensor like that disclosed in EP-0 182 921 B1.
For some time now, concentration of gaseous anhydrides has been determined using the Nernst equation in gas concentration cells with solid electrolytes, said electrolytes consisting of gas-tight, sintered pure doped alkaline or alkaline earth salts. Depending on the type of gas, solid electrolytes based on Na.sub.2 CO.sub.3 are used for CO.sub.2 sensors, those based on NA.sub.2 SO.sub.4 for SOx sensors, and those based on Ba(NO.sub.3).sub.2 for NOx. Such systems as is known permit measurements to be conducted with long-term stability if the solid electrolyte remains gas-tight. However, a reference gas with a constant oxygen and anhydride concentration is required, said gas being expensive to handle and produce industrially. There has been no lack of attempts to solve the problem of long-term gas tightness of the electrolyte and to produce a reference electrode that is easier to handle.
As is known, gas tightness can be improved for example by adding aliovalent cations to the alkaline salts which simultaneously increases conductivity (DD-PS 235 335 A1). Such systems however also generally have the disadvantage that the measuring and reference gas chambers must be separated from one another in a gas-tight manner, so that they cannot be manufactured economically, for example, by screen printing.
Nonetheless, systems with reference gas chambers are constantly under development, with EP-0 470 625 A2 proposing for example that a reference electrode layer made of Pt be applied to a heatable substrate and that a solid electrolyte, made of Nasicon for example, be placed on top of it, said electrolyte being covered on its upper surface by a measuring electrode, with a gas reference chamber being sealed off between the solid electrolyte and the substrate around the reference electrode layer by means of a glass covering that lies above the lateral surfaces of the substrate and the solid electrolyte, said electrolyte being completely enclosed as a result. Such an arrangement is once again not industrially effective, for example cannot be manufactured by the screen-printing process. It is also difficult to produce stable connections between the solid electrolyte material and the glass without reactions occurring between the two materials with time.
The problem of separating the measuring and reference gas chambers can be avoided by using a second solid electrolyte. Solid body cells have been proposed (German patent 40 22 136 C2) for measuring gaseous anhydrides such as CO.sub.2 for example, said cells consisting of a combination of alkaline-ion-conducting solid electrolyte, formed from the anhydride, from NA.sub.2 CO.sub.3 for example, and a ceramically stable usually dense material, Nasicon or .beta.-Al.sub.2 O.sub.3 for example, in which the same cation is mobile. Both solid electrolytes are provided with metal coatings with respect to which electrochemical equilibria can be created as a consequence of which an equilibrium cell potential or e.m.f. can be measured between the latter. In such arrangements, the electrochemical reaction must take place at the measuring electrode of the anhydride, for example, CO.sub.2 and O.sub.2, while only oxygen is effective on the reference side. The oxygen species that are then formed in the alkaline-ion-conducting ceramic solid electrolyte cannot migrate through the electrolyte so that interference with the electrode potential caused by the measuring current or gas exchange cannot be ruled out. Such problems make themselves felt in the form of long adjusting times and drift phenomena of the cell signal.
If a solid electrolyte that conducts oxide ions is used instead of one that conducts alkaline ions, a loadable oxygen reference electrode can be produced that forms the interface between this electrolyte and the one formed from the anhydride, but is gas-sensitive and must therefore be separated in a gas-tight manner from the measuring gas chamber, which in turn is not easy to accomplish.
All efforts to produce a reference electrode on solid electrolytes that conduct alkaline ions had in common the fact that an attempt was made to create conditions for the electrode reaction that were as reversible as possible. According to general understanding, such an electrode can only be one in which alkaline ion passage takes place through the phase boundary. Problems caused by current flow are compensated in that an alkaline ion current passes the phase boundary depending on the current direction and the required electrons come from the metal conductor, gold for example, or are given up to the latter.
The other possibility involves eliminating a metal/metal ion electrode (for example Na/Na+ electrode) by providing an O.sub.2, metal/ceramic alkaline conductor electrode (called oxygen electrode for short). The disadvantage of such an oxygen electrode is that it can easily be disrupted even when sufficient oxygen enters because only a small amount of oxygen can react during the current flow and the conditions that block the electrode reaction become established very quickly. This electrode reaction can be disrupted by an electrochemical reaction with anhydrides that takes place in parallel when oxygen is present, so that mixed potentials can form as a result of the degree of interference.
The use of a metal/metal ion electrode for example sodium (1)/Na+ conducting solid electrolyte as the stable reference electrode like that proposed by J. Liu and W. Weppner in Solid State Comm. 76(1990), pages 311-313, is theoretically possible but neither safe nor easy to do (owing to the sodium which liquefies at measuring temperatures), so that such electrodes are not considered for mass application.
Improved stability of the reference electrode can be achieved by using systems which are both alkaline ion conducting and electron conducting. Alloys of noble metals and alkaline metals, for example sodium in gold, however are not very stable chemically at the measuring temperatures. In addition, the alkaline metal activity changes readily which in turn leads to a change in potential. Alkaline compounds of transition metal oxides, such as tungsten bronzes, in which the alkaline ion often occurs intercalated, usually have a greater phase width for alkaline ions and are also frequently poisonous. They must exhibit good adhesion and be applied to the solid electrolytes while protected from the measuring gas. The coating materials react with these systems in the long term, which can result in crack formation and thus the entry of measuring gas to the reference electrode and thus cause drift phenomena. The application of the reference and coating materials poses technical difficulties, said materials requiring adjustment to the solid electrolyte in terms of their coefficients of expansion so that such sensors are not simple to manufacture, for example, by screen printing.
It has also been proposed to coat the entire solid electrolyte not covered by the metal salt, said electrolyte having on opposite sides a reference electrode made of metal and a measuring electrode made of metal with the metal salt layer on top, with an electrically insulating cover layer made of ceramic, glass, or plastic (EP-0 182 921 B1). However, this still does not eliminate the disadvantages that result from the necessary combination of different materials. There is also the danger, as our own investigations have shown, that in the long term, the cover layer reacts with the solid electrolyte or, as a result of the impurities penetrating into the cover layer, the layer becomes more or less conductive at the operating temperature of the sensor and thus causes a diffusion potential and thus a drift in the sensor signal. Especially the glasses proposed for the cover layer can become electrically conducting through the inward diffusion of alkaline ions at the phase boundary. All in all, it was not possible in the tests that were conducted either to produce a stable boundary surface between the metal salt and the cover layer or a stable interface between the electrolyte and the cover layer.
Consideration was also given to using a metal/metal oxide mixture (for example Pd/PdO) with a thermodynamically defined constant oxygen partial pressure. The disadvantage of such electrodes is the temperature dependence of the oxygen partial pressure that becomes established. In addition, there are also the disadvantages associated with the gas-tight covering of such systems.